1. Field of the Invention
The present invention relates to an electron gun for a color picture cathode-ray tube (hereinafter "CRT") for reducing deterioration of focusing property, minimizing deflection aberration from deflection yoke, and improving focusing property.
2. Description of the Prior Art
Generally, conventional electron guns are utilized in an electrostatic focusing system. An electrostatic focusing lens of the electrostatic focusing system placed between a first accelerating and focusing electrode and a second accelerating and focusing electrode closely focuses the beams from the electron beam forming region consisting of cathodes and a number of electrodes in front of the cathodes. The performance of such electrostatic lens depends on the difference of focusing force between the near-axis region and the maximum outer angle region, causing the spherical aberration of the lens. The larger the lens diameter is, the lesser the spherical aberration becomes.
In order to obtain a good electron beam focusing property for an electron gun for the color picture CRT, the electron beam through-holes of the first and second accelerating and focusing electrodes are preferred to be as large as possible.
FIGS. 1, 2(A), 2(B), and 3 show a conventional electron gun for a color picture cathode-ray tube. Such conventional electrode gun includes a first accelerating and focusing electrode 8 and a second accelerating and focusing electrodes.
The first accelerating and focusing electrode 8 and the second accelerating and focusing electrodes 9 form an electrostatic lens. Each of the electrodes 8 and 9 has an open end on one side and a oblong-shaped closed end and the two closed ends face each other. The closed end 13 of the electrode 8 and the closed end 14 of the electrode 9 connected to respective end walls 11 and 12, respectively are provided with three through-holes 11a and 12a, 11b and 12b, and 11c and 12c for passing electron beams. The through-holes have a rimmed lip extending from the closed end faces, respectively.
In the focusing electrodes 8 and 9, the upper walls 11 and 12 have 6 mm in height (hereinafter "H"), the lips 15 and 16 have 1.2-3.5 mm h, the electron beam holes have 5.5-5.9 mm in diameter (hereinafter "D"), and the distance between the adjacent holes is 6.6-6.9 mm. However, the above measurements are subject to the limitation of the optimum diameter 29.1 mm of the tube neck in a conventional color picture CRT.
The first and second focusing electrodes 8 and 9 are also arranged, as shown in FIG. 1, for the first focusing electrode 8 to be connected with its open end to a third grid electrode 7. The electron gun for the color CRT basically includes a cathode 4 fixed to a cathode support 2, first, second and third grid electrodes 5, 6, and 7, and the first and second accelerating and focusing electrodes 8 and 9 wherein they are arranged in a pile in the above mentioned order and fixed to a pair of bead glass 10.
In a conventional color picture CRT shown in FIG. 1, a heater 3 welded to a support 1 and inserted into the cathode 4, heats the cathode 4 and make it emit heated electrons. The third grid electrode 7 having an elongated cylindrical configuration and positioned in front of the first and second grid electrodes 5 and 6 connects to the first accelerating and focusing electrode. The first and second accelerating and focusing electrodes 8 and 9 constitute an electrostatic focusing lens, that is a bi-potential focus (hereinafter "BPF").
The first and second accelerating and focusing electrodes 8 and 9 may be utilized in an electron gun including a plurality of additional electrodes disposed in the third grid electrode 7.
According to the conventional electron gun for the CRT, the electrons emitted from the cathode 4 by the heating of the heater 3 form an electron beam. The electron beam passes through the first grid electrode 5, the second grid electrode 6 and the third grid electrode 7 and enters the electrostatic focusing lens formed between the first and the second focusing electrodes 8 and 9. The received electron beams are closely focused to reach the fluorescent screen of the CRT and form a beam spot. The beam spot formed on the screen should have a high density in a round form in the least possible area.
However, in the first and second accelerating and focusing electrodes 8 and 9 for forming an electrostatic focusing lens of the electron gun shown in FIG. 2, the beam spot is distorted into a laterally oblong shape under the influence of the electrostatic focusing lens diameter. The diameter is restricted by the limited holes 11a-11c and 12a-12c for passing electron beams. Furthermore, the beam spot is distorted the deflection aberration caused by a deflection yoke. Therefore, the beam spot has a low density which deteriorates the resolution of the color picture CRT as a disadvantage.
For example, as shown in FIG. 3, the electrodes 8 and 9 for constituting an electrostatic focusing lens are housed in a tube neck 17 having an optimum diameter of 29 mm for the CRT. The thickness (b) of the rims respectively surrounding three beam through-holes in the closed end face of the first focusing electrode 8 has to be 1 mm in actual structure. Therefore, their relation is expressed by the following formula (I): EQU D.ltoreq.S-1 (I)
Furthermore, the distance (a) between the inner wall of the tube neck 17 and the outer end walls 11 and 12 of the focusing electrodes requires to be 1 mm, their relationship being expressed by the following formula (II): EQU D.ltoreq.R-(2a)-2(S+b) (II)
wherein R is the inner diameter of the tube neck, approximately 24 mm.
Therefore, the diameter is represented by the following formula (III): EQU D.ltoreq.20-2S (III), and
Dmax=6 mm and Smax=7 mm result from the formulas (I) and (III).
The conventional first and second focusing electrodes 8 and 9 form merely an electrostatic focusing lens of 6 mm at the maximum in diameter. Therefore, the small diameter of the focusing lens increases the spherical aberration, that is, the difference in focusing force between the near-axis region and the maximum outer angle region in the lens forms beam spots with a low density on the screen.
Also, because of the round shape of the electrostatic focusing lens, the beam spot with a low beam density distorts into a laterally oblong shape by the deflection aberration of deflection yoke to and further deteriorates the resolution of the color picture CRT. The known art concerning the lateral distortion of electron beams by the deflection aberration of deflection yoke will be omitted.
Besides, in order to obtain a better concentration of three electron beams for focusing three beam spots to gather a small converging area on the image screen, the distance S between adjacent beam holes is required to be smaller, but the conventional art gives 7 mm of S at the maximum under the limitation of the maximum lens diameter of 6 mm from the formulas (I) and (III). Accordingly, it is an disadvantage that the large distance S between the holes brings deterioration of the concentrating property of the CRT.